Microscope with rotating beam system

ABSTRACT

A microscope comprising a coherent light source producing a coherent light beam, a light beam guide system comprising a beam splitter configured to split the coherent light beam into a reference beam and a sample illumination beam, a sample holder configured to hold a sample to be observed, a sample illumination device configured to direct the sample illumination beam through the sample and into a microscope objective, a beam reuniter configured to reunite the reference beam and sample illumination beam after passage of the sample illumination beam through the sample to be observed, and a light sensing system configured to capture at least phase and intensity values of the coherent light beam downstream of the beam reuniter.

This application is a divisional application of U.S. patent applicationSer. No. 15/513,012 filed on Mar. 21, 2017, which is the U.S. nationalphase of International Application No. PCT/IB2015/057195 filed on Sep.18, 2015, which designated the U.S. and claims priority to EP PatentApplication No. 14185718.5 filed on Sep. 22, 2014, the entire contentsof each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a microscope, in particular amicroscope for three dimensional tomographic imaging of biologicalmatter, including cells and microorganisms. The present invention mayalso be used more generally in the field of three dimensionaltomographic imaging of non-biological transparent materials.

BACKGROUND

Microscopes capable of three-dimensional imaging of biological cells andtheir internal structures are generally based on digital tomographictechniques and are often based on the use of marker dyes to enhance theintensity contrast between components of the cell. Marker dyes howevermay affect the matter to be observed, particularly in the case of livingcells, and also render the procedure more complex. A marker freenon-invasive microscopy method based on 3D refractive index computationis described in international patent application PCT/IB2011/051306. Thecapture of the image data needed to compute a refractive index basedimage of very small objects with sufficient detail of the internalstructure requires however a microscope that is very precise. Theprecision of conventional microscopes depends on the quality of thelenses and very low manufacturing tolerances in the assembly of thevarious components of the microscope. This leads to a costly microscope.Moreover, in order to have a high numerical aperture in order toincrease the resolution of the captured data, the available workingdistance and space for the sample to be observed is very limited. Thiscomplicates the preparation of the specimen or sample for viewing by themicroscope and limits the type of samples that may be observed and theforms in which they may be presented.

SUMMARY OF THE INVENTION

An object of the invention is to provide that a microscope that is easyand economical to implement and to use.

It is advantageous to provide a microscope that is accurate and offers ahigh resolution, yet that is economical to produce.

It is advantageous to provide a microscope that simplifies thepreparation of biological samples to be observed.

It is advantageous to provide a microscope that can be used with variousstandard or common forms of biological sample containers, includingculture dishes for living cells.

It is advantageous to provide a microscope that simplifies thepreparation of biological samples to be observed.

It is advantageous to provide a microscope that is reliable.

It is advantageous to provide a microscope that is versatile.

Disclosed herein is a microscope comprising a light source producing alight beam that is at least partially collimated, a light beam guidesystem comprising a beam splitter configured to split the light beaminto a reference beam and a sample illumination beam, a sampleobservation zone configured to receive a sample to be observed, a beamreuniter configured to reunite the reference beam and sampleillumination beam after passage of the sample illumination beam throughthe sample observation zone, and a light sensing system configured tocapture at least phase and intensity values of the light beam downstreamof the beam reuniter. In an embodiment the light source may beconfigured to generate a coherent or partially coherent collimated lightbeam, in particular a laser beam, for instance a diode laser beam. In avariant the light source may generate incoherent light, for instancewhite light, that is collimated for instance by means of an opticallens.

According to a first aspect of the invention, the light beam guidesystem comprises direction change mirrors to direct the reference beamand the sample illumination beam along their respective optical paths,wherein at least one of the direction change mirrors is a pivotallyactionable mirror controllable by an electronic control system of themicroscope. The control system is configured to generate a mirror anglecontrol signal to automatically control an angle of said at least onepivotally actionable direction change mirror, the mirror angle controlsignal being based at least partially on a signal of the light beamreceived, by means of a feedback loop of the control system, by thelight sensing system.

In an embodiment, the control signal may be based on both the phase andintensity values of the light beam measured by the light sensing system.

Advantageously, the microscope may be easily calibrated aftermanufacturing or transport, at regular interval or before each use, byan electronic control of the angle of one or more of the directionchange mirrors in the sample illumination and/or in the reference beampath.

According to a second aspect of the invention, the sample illuminationdevice comprises a mirror system configured to direct the sampleillumination beam at a non zero illumination angle (non orthoscopicillumination) with respect to an optical axis, and a rotating beammechanism configured to rotate the angled sample illumination beam atleast 360° (2 pi radians) around the optical axis.

The rotating sample illumination beam with a pre-determined illuminationangle according to the invention is particularly advantageous overconventional solutions in that it allows to illuminate the microscopicobject at a large illumination angle, limited principally by thenumerical aperture of the microscope objective, in an arrangement thatis low cost compared to lens based solutions. In effect, the rotatingbeam does not change the beam shape as opposed to lenses or refractiveor diffractive elements which thus need to be manufactured withextremely high quality in order to reduce beam shaping, thus increasingproduction costs as opposed to the solution provided by the presentinvention. Moreover the rotating beam allows a large working space forthe sample thus providing versatility in the size of the samples thatmay be positioned on the sample holder and viewed by the microscopewhile significantly reducing the manufacturing costs of the microscopeand sensitivity to the quality of the optical elements along the opticalpath.

According to a third aspect of the invention, the microscope furthercomprises an optical path difference (OPD) adjustment device configuredto adjust the optical path length of the reference beam relative to thesample illumination beam, the OPD adjustment device comprising a firstlight deviating element and a second light deviating element, eachmounted on pivot supports configured to vary the angle of the firstlight deviating element relative to the second light deviating element,whereby the angles of inclination of the light deviating elementsinfluences at least the optical path difference.

A continuous and accurate adjustment of the optical path difference maythus be achieved. Moreover, if needed, a dynamic adjustment is possible.

In an embodiment, the mirror system of the sample illumination device ismounted in a rotating support and the rotating beam mechanism is formedby the rotating support and a motor drive to rotate the support.

In an embodiment, the rotating beam mechanism comprises rotating tiltactionable mirrors to direct the sample illumination beam on the mirrorsystem of the sample illumination device and wherein the mirror systemis mounted on a fixed support.

In an advantageous embodiment, the pivotally actionable mirror is aMicroelectromechanical (MEMS) type component.

In an advantageous embodiment, the pivotally actionable mirror ispositioned essentially above the sample in line with the optical axis ofthe microscope objective.

In an advantageous embodiment, the direction change mirrors comprise atleast first and second direction change mirrors arranged in the opticalpath of the sample illumination beam downstream of the beam splitter,both mirrors being pivotally actionable to correct for optical patherrors or to change the sample illumination angle.

In an advantageous embodiment, the microscope further comprises a dataprocessing system configured to receive a plurality of image frames datafrom the light sensing system, said plurality of image frames beinggenerated for at least a 360° rotation of the sample illumination beamaround the microscope objective optical axis.

In an advantageous embodiment, the number of frames per 360° captured bythe light sensing system and data processing system is greater than 10,preferably greater than 20, more preferably greater than 30.

In an advantageous embodiment, the image frames data are reconstitutedby the data processing system, or supplied by the data processing systemto an external computing system, for processing into a three dimensionalimage of the microscopic object.

In an advantageous embodiment, the microscope is configured to generatea three-dimensional image of the microscopic object based on therefractive index distribution of the microscopic object by determiningthe phase shift of the sample illumination beam after passing throughthe microscopic object.

Also disclosed herein is a method of controlling a microscope comprisinga light source producing a light beam, a light beam guide systemcomprising a beam splitter configured to split the light beam into areference beam and a sample illumination beam passing through the lightbeam guide system directed by at least one direction change mirror beingpivotally actionable (TM1, TM2, TM3, TM4) to guide the reference beamand sample beam along their respective optical paths, a sampleobservation zone configured to receive a sample to be observed in a pathof the sample illumination beam, a beam reuniter configured to reunitethe reference beam and sample illumination beam after passage of thesample illumination beam through the sample observation zone, a lightsensing system configured to retrieve at least phase and intensityvalues of the light beam downstream of the beam reuniter, and anelectronic control system, the method characterized by:

-   -   receiving through a feedback loop in the control system a signal        generated by the light beam from the light sensing system,    -   generating in the control system a mirror angle control signal        based at least partially on said signal received from the light        sensing system, and    -   transmitting the mirror angle control signal to said at least        one pivotally actionable direction change mirror to control an        angle of the pivotally actionable direction change mirror.

In an embodiment, the method comprises rotating the sample beam relativeto the sample.

In an embodiment, the generated mirror angle control signal is dynamic.

In an embodiment, the mirror angle control signal is generateddynamically as a function of an angle of rotation of the sample beamrelative to the sample.

In an embodiment, the generated mirror angle control signal is static.

In an embodiment, the signal received from the light sensing system andgenerated by the light beam on which the mirror angle control signal isbased includes any one or more of intensity, coherence, fringe frequencyand phase of the light beam received by the light sensing system.

In an embodiment, the method further comprises receiving a plurality ofimage frames data in data processing system of the microscope from thelight sensing system, said plurality of image frames being generated forat least a 360° rotation of the sample illumination beam around themicroscope objective optical axis.

In an embodiment, the number of frames per 360° captured by the lightsensing system and data processing system is greater than 10.

In an embodiment, the image frames data are reconstituted by the dataprocessing system, or supplied by the data processing system to acomputing system, for processing into a three dimensional image of themicroscopic object

In an embodiment, image frames data are further employed by the dataprocessing system, or supplied by the data processing system to acomputing system, for estimating optical properties of the sample toimprove the three dimensional image of the microscopic object.

In an embodiment, the method further comprises generating athree-dimensional image of the microscopic object based on therefractive index of sections of the microscopic object by determiningthe phase shift of the sample illumination beam after passing throughthe microscopic object.

Also disclosed herein is a method of controlling a microscope, themethod characterized by dynamically adjusting an optical path length(OPL) of the reference beam by the control system of the microscope tokeep an optical path difference (OPD) between the reference and samplebeams below a coherence length of the light source, the methodcomprising the following steps:

a) position the rotating beam system in a first position,

b) position said at least one pivotally actionable direction changemirror (TM3, TM4) configured to direct the reference beam in a firstposition,

c) measure a position of the reference beam signal captured by the lightsensing system while the sample beam is switched off,

d) switch on the sample beam and measure a fringe contrast of a signalcaptured by the light sensing system,

e) change by an increment the position of said at least one pivotallyactionable direction change mirror (TM3, TM4),

f) repeat steps c) to e) until the sum of increments corresponds to apre-defined working range of the pivotally actionable direction changemirror (TM3, TM4),

g) compare the fringe contrast measurements obtained for each incrementand store in look-up table (LUT) of a memory of the control system theposition of the pivotally actionable direction change mirror (TM3, TM4)for the fringe contrast measurement with the highest value, inconjunction with the position of the rotating beam system;

h) rotate by a small increment the rotating beam system and repeat stepsb) to g) until the rotating beam system has completed a 360° rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantageous features of the invention will beapparent from the claims, from the detailed description, and annexeddrawings, in which:

FIG. 1a is perspective view with a portion of housing removed of amicroscope according to an embodiment of this invention;

FIG. 1b is a cross-sectional view through the microscope of FIG. 1 a;

FIG. 1c is a schematic simplified diagram of the configuration of amicroscope according to an embodiment of this invention based on aMach-Zehnder scheme;

FIG. 1d is cross-sectional view of a microscope according to anotherembodiment of this invention;

FIG. 1e is a cross-sectional perspective view of the microscope of FIG.1 d;

FIG. 2a is a schematic simplified diagram of the configuration of amicroscope according to an embodiment of this invention;

FIG. 2b is a schematic diagram similar to FIG. 2a illustrating theeffect on the optical path of various defects such as manufacturingtolerances, wear, vibration or heat;

FIGS. 2c and 2d are schematic diagrams of the configuration of a part ofthe microscope illustrating the effect of a mechanical defect on theoptical path length during a revolution of a rotating scanning arm ofthe microscope, whereby FIG. 2c shows a scanning head without defect andFIG. 2c shows a scanning head with mechanical defect (misalignment);

FIGS. 3a and 3b are partial views of samples according to first andsecond variants on a sample holder of a microscope according to thisinvention;

FIG. 4a is a view of an optical path difference (OPD) adjustment device,based on a transmissive principle, of a microscope according to anembodiment of this invention;

FIG. 4b is a simplified schematic diagram of the optical path device ofFIG. 4a illustrating OPD walk-off correction;

FIG. 4c is a view similar to FIG. 4b variant illustrating OPDcompensation;

FIG. 4d is a detailed schematic view of a transparent (e.g. glass) slabof an OPD device showing various angles and dimensions discussed in thedetailed description;

FIG. 4e is are graphs showing the relation of the optical path lengthvariation versus the angle of orientation of the transparent slabs ofthe OPD device of FIG. 4 a;

FIGS. 4f to 4h are schematic illustrations of a transmissive OPD deviceaccording to an embodiment of the invention showing perfectsynchronization (FIG. 4f ), imperfect synchronization (FIG. 4g ) andcorrection of imperfect synchronization by means of tiltable mirrors(FIG. 4h );

FIG. 4i is a diagram illustrating the steps in a control procedure foradjustment of the OPD device of FIG. 4 a.

FIG. 5a is a schematic simplified diagram of the configuration of amicroscope comprising an OPD device based on a reflective principle,according to an embodiment of this invention;

FIGS. 5b, 5c, and 5d are schematic simplified diagrams of variants ofthe OPD device of the embodiment of FIG. 5 a;

FIG. 5e is a diagram illustrating the steps in a control procedure foradjustment of the OPD device of FIG. 5 a;

FIG. 5f is a diagram illustrating the steps in a control procedure fordynamic adjustment of the OPD device of FIG. 5 a;

FIG. 5g is a graph illustrating the optical path length (OPL) differenceof the OPD system as a function of the scanning head position allowingto determine the optimum position;

FIGS. 6a and 6b are simplified schematic views illustrating inclinedillumination of a sample to be observed, FIG. 6a illustrating effect ofa variation in height (Δh) of the liquid in which the sample is immersedand FIG. 6b illustrating effect of a variation in the illumination angle(α);

FIG. 6c is a simplified schematic view illustrating inclinedillumination of a sample to be captured with means to correct effects ofa variation in height of the liquid in which the sample is immersed;

FIGS. 7a, 7b, 7c and 7d are simplified schematic illustrations of arotating illumination beam system relative to a sample according todifferent variants of a microscope according to this invention;

FIG. 8 is a perspective simplified view of a MEMS based mirror withadjustable tilt angle for use in the microscope according to embodimentsof this invention;

FIG. 9 is a flowchart diagram illustrating steps of a calibrationprocess (P1) of a microscope according to embodiments of this invention,in which an angle of a tiltable mirror in a sample illumination beampath is adjusted;

FIGS. 10a and 10b are flowchart diagrams illustrating steps of afeedback loop control process (P2) for sample based automated phasecorrection of a microscope according to embodiments of this invention;

FIG. 11 is a flowchart diagram illustrating steps of a calibrationprocess (P5) of a microscope according to embodiments of this invention,in which an angle of a tiltable mirror in a reference and sampleillumination beam path is adjusted;

FIG. 12 is a flowchart diagram illustrating steps of a qualityassessment process (P6) of a microscope according to embodiments of thisinvention, in which an angle of a tiltable mirror in a reference beampath is adjusted;

FIG. 13 is a flowchart diagram illustrating steps of an error analysisprocess (P7) of a microscope according to embodiments of this invention;

FIG. 14a is a schematic illustration of a Fourier transform of theintensity pattern of a hologram captured by a light sensing system of amicroscope according to embodiments of the invention;

FIG. 14b is a schematic illustration of different levels of feedbackcontrol of the tiltable mirrors of a microscope according to embodimentsof this invention;

FIG. 15a is an illustration of diagrams of aberrations of differentorders captured by the light sensing system of a microscope according toembodiments of the invention, for use in amplitude and phase analysis ofthe beam received by the light sensing system;

FIGS. 15b and 15c are graphical representations of a signal generated byaberrations of different orders captured by the light sensing system ofa microscope according to embodiments of the invention;

FIG. 16a is an illustration of geometrical angles and lengths of asample illumination beam at an interface between matter of two differentrefractive indices n1, n2, for illustration of a sample immersed in aliquid of height h;

FIG. 16b is an illustration of the relationship between intensity of themeasured beam signal and a defocus angle (Δα);

FIG. 16c is an illustration of the relationship between the defocusangle (Δα) of the sample illumination beam and the height of the sampleliquid to determine the phase correction (ϕ);

FIG. 16d is an illustration regarding a sample-thickness h1, h2dependent walk-off of the sample illumination beam from the microscopeobjective's field of view (FOV);

FIG. 17a is a schematic diagram of a sample illumination beam path toillustrate angles and dimensions and FIGS. 17b -1, 17 b-2, and 17 b-3are tables of results of the varying angles and dimensions to illustratethe effects of tolerances in fabrication, vibration or thermal effectson the angle of one or more tiltable mirrors in the sample illuminationbeam path needed to compensate for the tolerance.

FIG. 18 is a flowchart diagram illustrating overall the implementationof the various procedures P1 to P7 in the operation of a microscopeaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring to the figures, starting in particular with FIGS. 1a to 2b ,an exemplary embodiment of a microscope 2 comprises a light source 4 inparticular a coherent light source such as a laser beam device, a lightbeam guide system 6, a light sensing system 8, a data processing system10, in particular for processing generated image data, an electroniccontrol system 15, and a housing and support structure 12 generallyhousing and supporting various components of the microscope 2.

The microscope 2 is capable of three-dimensional imaging of biologicalcells and microorganisms, including living cells. The images of cellsand other samples of biological micro-matter may be generated using thetechniques described in international patent applicationPCT/IB2011/051306 which is incorporated herein by reference.

The housing and support structure 12 comprises a sample observation zoneincluding a sample holder 18 configured for holding biological samplesin various standard and non-standard formats. The sample holder has aposition adjustment mechanism allowing the position of the sample to bemoved three-dimensionally, in particular a height adjustment and anadjustment in the plane traversed by the sample illumination light beam.A position adjustment mechanism for a microscope is per se well knownand allows the position of the sample to be observed to be adjustedrelative to the microscope objective position below (transmission) orabove (reflection) the sample.

In a variant (not shown), the sample observation zone may have a conduithaving a transparent section extending through the zone traversed by thesample illumination beam, configured to observe sample material suppliedby flow of liquid to the illumination beam path in a closed conduit.

Referring to FIGS. 3a and 3b , biological samples may be provided in aclosed containing system 3 a, or in an open dish 3 b (such as a petridish). The biological samples may for instance be provided according tothe following non-limitative examples. As a general requirement for anytype of sample, the buffering medium 5 should not scatter incominglight. Clear liquids such as phosphate buffered saline (PBS) or4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) may be usedfor instance. In order to keep cells alive for 3 to 4 hours, sampleswith PBS with for instance glucose (e.g. 25 mmol) and HEPES (e.g. 10mmol) work well. For static experiments, cells fixed withParaFormaldeHyde (PFA) may be usable for several weeks. The observationmay be made through coverslips of typically 170 microns thick (mostmicroscope objectives are optimized for such coverslips), and due to thelimited working distance of such objectives, cells should be fixed onthe coverslip preferably not much further than 30 microns away from it.Optical surfaces should be as clean as possible and cells holders shouldbe carefully cleaned, at least twice by experience, so that as few deadcells or any kind of remains as possible are floating in the mountingmedium. In the example of FIG. 3a , the buffering medium 5 is sealed ina chamber to avoid liquid drying out or leakage. A seal 7, for instancein the form of a tape spacer (for instance from Grace Bio-LabsSS1X9-SecureSeal Imaging with inner diameter 9 mm and thickness 0.12 mm)is mounted between the coverslip 9 and base 11.

For living cells provided in a standard Petri dish 3 b with atransparent base, the cells can be directly observed by locating thebase of the Petri dish against or very close to the microscope objective37. The amount of liquid is not important, provided that the bottomsurface of the dish against which cells are located, and which forms theplane of observation 13, is covered.

The confluency of cells is preferably greater than 20% in order toensure easy location of a cell for observation.

The coherent light source 4 may be in an exemplary embodiment a laserbeam generator, for instance of the type diode laser beam at 520 nanometer wave length. Laser beams of other wave lengths may also be usedbeing noted that the shorter of the wave length the higher the possibleresolution.

The light sensing system 8 may in particular be a camera with an imagesensor configured to read the light beam received after passing throughthe sample and transmitting the detected light signal to the dataprocessing system 10. The camera may be of a known type with CCD, CMOSor other types of photo sensors capable of picking up the wave length,phase and intensity of the received light beam and transmitting thisinformation to the data processing system 10.

The light beam guide system 6 is configured to divide the coherent lightbeam into two beams, a reference beam 7 a and a sample illumination beam7 b that follow different paths, respectively a reference beam opticalpath 22 and a sample beam optical path 20. The light beam guide systemcomprises a beam splitter 14 that receives the coherent beam 7 from thecoherent light source 4 and spits it into the two beams 7 a, 7 b. Thebeam splitter is per se a well known device and need not be furtherexplained herein.

After the sample illumination beam 7 b has passed through the sample 1and before being captured by the light sensing system 8, the sampleillumination beam 7 b and the reference beam 7 a are reunited by a beamreuniter 16. The beam reuniter, which may have the same configuration asa beam splitter operating in reverse mode is also per se well known andneed not be further described herein. The splitting of the coherent beam7 along two paths, one passing through the sample, allows measuring aphase shift of the sample illumination beam 7 b relative to thereference beam 7 a, dependent on the refractive index of the section ofsample matter through which the sample illumination beam 7 b is passingthrough.

The reference beam optical path system 22 comprises direction changemirrors 30 to redirect and guide the reference beam 7 a along its path,and an optical path difference (OPD) adjustment device 32.

Referring to FIGS. 4a to 4c , an OPD adjustment device 32 according tothe illustrated embodiment is based on a transmissive principle andcomprises a first light deviating element 42 mounted on a pivot support46, and a second light deviating element 44 mounted on a second pivotsupport 48. The light deviating elements 42, 44 are made of atransparent material with a refractive index greater than air, forinstance a glass slab, configured to bend the reference light beampassing therethrough in a manner to allow adjustment of the length ofthe optical path of the reference beam. The transparent material may bemade of glass or a polymer or other transparent solids, and may simplyhave the shape of a flat plate or slab, or may have curved ornon-parallel outer surfaces. The pivot supports 46, 48 which comprise apivot axis P1, P2 and may be coupled together mechanically byinterengaging teeth 50 such that the pivot supports rotatesimultaneously and in opposite angular directions w1, w2. The lightdeviating elements 42, 44 mounted on the pivot supports thus pivotsimultaneously and in opposite angular directions to adjust the lengthof the optical path of the beam 7 a. The greater the angle Ω between thefirst and second light deviating elements 42, 44, the longer the opticalpath of the reference beam 7 a. The pivot supports 46, 48 mayadvantageously be micro machined parts made from a semi conductingsubstrate using MEMS manufacturing techniques, the rotation of the pivotsupports being controlled by inductive current flowing in a section ofthe semi-conductor.

In an advantageous embodiment, at least one of the light deviatingelements 48 may be rotatably mounted on its pivot support in order tohave an additional independent rotation of angle Ω, with respect to theother light deviating element, configured to allow adjustment foroptical path misalignment (walk-off) due to misalignments, manufacturingtolerances, and the like.

In an alternative embodiment, the first and second pivot supports may beindependently controlled and not mechanically directly coupled in orderto allow adjustment of both optical path length and optical pathwalk-off.

In a variant, as schematically illustrated in FIG. 4c , at least some ofthe direction change mirrors 30 of the reference beam path system 20 arepivotable or tilt adjustable to adjust optical path length and/oroptical path misalignment (walk-off).

Referring to FIGS. 4d and 4e , the principle of the OPD compensationsystem according to the above described embodiment is further described.It is known that a glass slab may be used to laterally shift a beam andincidentally increase the optical path length. The lateral shift mayhowever advantageously be compensated by the use of two glass slabsrotated one clockwise and the other one counterclockwise, to have boththe lateral shift compensated while the OPD twice as large.

Referring to FIGS. 4d to 4f , the lateral shift (C′D) and the OPDnamely, OPD=(BC′−BC), can be expressed as follows:

$\overset{\_}{C^{\prime}D} = {{e \cdot \sin}\;{\theta_{1}\left( {1 - \frac{\cos\;\theta_{1}}{\sqrt{n^{2} - {\sin^{2}\theta_{1}}}}} \right)}}$${OPD} = {{{ne}\left( {\frac{1}{\cos\;\theta_{2}} - 1} \right)} - {e \cdot \left( {\frac{\cos\left( {\theta_{1} - \theta_{2}} \right)}{\cos\;\theta_{2}} - 1} \right)}}$

To compensate the lateral shift of the beam, another glass slab istilted by the opposite angle: the shift is compensated and the OPD isdoubled at the same time as illustrated in FIG. 4f . FIG. 4f shows theOPD compensation results according to the above calculation for twoslabs of 10 mm thick each, of RI 1.5 and tiltable from 0 to 60°.

The OPD adjustment device according this embodiment may be implementedinto the microscope to adapt the optical path to the sample thicknessbetween two mirrors 30 as shown in FIG. 4g . Due to an error of command,a difference of glass slab thickness e, or simply loose tolerance in theassembly of components in the support structure 12, the exitingreference beam 7 a will be laterally shifted as shown in FIG. 4h . For adistance of 300 mm between the lens and the camera and a focal length of20 mm for the lens, if one admits a possible shift of a tenth of thefield of view on the sensor, i.e. around 500 μm, it means a maximumadmissible lateral shift of about 40 μm on the second mirror. If weadmit such a maximum lateral shift, the two slabs 42, 44 should betilted by angles with a maximum difference of 0.25°, or the differencesof thicknesses of the slabs should not exceed 100 μm. To avoid imposingtight tolerances on the mechanical drives or the dimensions of the glassslabs, it is advantageous to replace the mirrors 30 by tiltable mirrorsTM3, TM4 controlled by a microcontroller of the control system 15—seeFIG. 4 i.

The procedure of command of the full OPD compensation system can bedescribed as follows with reference to FIGS. 4j and 4 h:

-   -   A sample 1 is placed in the sample observation zone 17 and ready        to be explored;    -   The glass slabs 42, 44 are placed in the neutral position (θ=90°        in FIG. 4h ).    -   The tiltable mirrors TM3, TM4 (for instance MEMS mirrors) are        controlled to best uniform the intensity on the sensor. To ease        the search, the sample illumination beam 7 b can be deflected        out of the aperture of the microscope objective 37;    -   The fringe contrast is evaluated from the calculation of the        carrier peak energy compared to the DC component energy in the        Fourier domain.    -   The OPD scanning starts by tilting the glass slabs 42, 44 in a        certain direction by a certain small angle;    -   For each increment of angle (the slabs can be driven by a        stepper motor with an appropriate gear reduction ratio), the        laser spot is kept centered on the light sensor through a PID        algorithm;    -   The sample illumination beam 7 b is redirected properly and the        fringe contrast is assessed;    -   Once the scan is complete, the slabs 42, 44 are properly        oriented in the position which yield the highest fringe        contrast.    -   The positions of the tiltable mirrors TM3, TM4 are refined.    -   The microscope is ready to optimally measure the sample 1.

In a variant, to speed up the iterative search described above, theoptical thickness obtained from procedure P1 (used in procedure P2) canbe used to define a starting point of the slab angles close to theexpected optimal position: the tilt angle of the slabs resulting in thehighest fringe contrast is directly dependent on the optical thicknessof the sample estimated in procedure P1.

Referring to FIGS. 5a to 5d , an OPD adjustment device 32 according tothe illustrated embodiment is based on a reflective principle andcomprises a first direction change mirror 30 a, a second directionchange mirror 30 b, and a third direction change mirror 30 c. At leastone of the first and third direction change mirrors 30 a, 30 c aretiltable, whereas the second direction change mirror 30 b, which ispositioned intermediate the first and third mirrors along the opticalpath of the reference beam, is translatable. Translation of the secondmirror 30 b changes the length of the optical path, the tiltable firstand third mirrors 30 a, 30 c being rotated to correct the direction ofthe reference beam as a function of the displacement of the secondmirror 30 b.

In the variant of FIG. 5b , the second mirror is mounted on a support(not shown) configured to move the second mirror in a direction obliqueto a direction orthogonal to the second mirror reflection plane (alsonamed angle bisector displacement), whereby both first and third mirrors30 a, 30 b are tiltable to adjust for the variation in angle of thereference light beam. In the variant of FIG. 5c , the second mirror ismounted on a support (not shown) configured to move the second mirrororthogonally to the second mirror reflection plane, whereby both firstand third mirrors 30 a, 30 b are tiltable to adjust for the variation inangle of the reference light beam. In the variant of FIG. 5d , thesecond mirror is mounted on a support (not shown) configured to move thesecond mirror in a direction parallel to the reference beam reflectedoff the second mirror (also named incircle displacement), whereby thefirst mirror 30 a is tiltable to adjust for the variation in angle ofthe reference light beam, and the second and/or third mirrors areoptionally tiltable.

An advantage of the reflective OPD embodiment compared to thetransmissive OPD embodiment is to minimize the error inducing effects ofoptical surfaces such as intensity variations or astigmatism, and to usethe degrees of liberty readily available in MEMS tilting mirror motions.Procedure P4 may be essentially equivalent to that of procedure P3,replacing slab rotation with mirror translation.

Referring mainly to FIGS. 1a to 2b , the sample beam optical path system20 also comprises direction change mirrors 26 in order to guide thesample illumination beam along its chosen path. The sample beam opticalpath system further comprises a sample illumination device 28 comprisinga mirror system 34 and a rotating mechanism 36. The mirror system 34comprises a first mirror 54 and a second mirror 56 configured to directthe sample illumination beam 7 b at a pre-determined angle α, withrespect to the axis A of the microscope objective 37, towards the sample1.

In an embodiment, the rotating mechanism 36 comprises a motor drive 38and a rotating support 40 the mirror system 34 being mounted in therotating support 40. In the present exemplary embodiment, the drive 38is coupled to the rotating support via a transmission comprising a belt39, however, many other transmission systems could be used to couple thedrive to the rotating support such as a gear system, or by magneticinduction coupling.

The rotating support 40 comprises a hollow or tubular axis 58 configuredto allow the sample illumination light beam to pass therethrough, and amirror support body 60 in which the first and second mirrors 54, 56 aremounted. The rotating support 40, in particular the hollow axis 58 ismounted via bearings 59 to the housing support structure 12.

The first mirror 54 redirects the light beam projected through thehollow axis onto the second mirror 56, and the second mirror redirectsthe light beam onto the sample 1 positioned on the sample holder 18 atan illumination angle α. The first and second mirrors may be arranged ina fixed relation within the mirror support body 60. In other variants,either the first mirror or the second mirror, or both, may be pivotallyadjustable. The pivotally adjustable first and/or second mirrors may beuseful to adjust the illumination angle to a chosen illumination angle,and/or to adjust for misalignment due to manufacturing tolerances, ordue to disadjustments during transport and handling or through use. Theillumination angle may also be adjusted by adjusting the angle of thedirection change mirrors 26 a, 26 b of the sample beam optical pathsystem 20. The angle adjustable mirrors 26 a, 26 b, can be micromachined parts in a form of MEMS fabricated mirrors formed out ofsemi-conducting material as illustrated in FIG. 8, as is per se known inthe art, to form mirrors with tilt angles that are electronicallyadjustable. The electronically angle adjustable mirrors thus allow tocompensate for manufacturing tolerances to calibrate the device, and toadjust illumination angles to chosen values either dynamically orstatically as needed. Auto calibration of the microscope is thuspossible after manufacturing and before each use to correct for anymisalignments, production tolerances, thermal effects, and wear thatdeviate the beam 7 b′ from the chosen path 7 b as best illustrated inFIG. 2 b.

The angle adjustable mirrors in the sample illumination path system mayadvantageously also enable static or dynamic adjustment for therefractive properties of the sample. For instance, a change in theheight delta h of the liquid (see FIGS. 5a to 5c ) within which thesample is immersed may be corrected for. Dynamic correction may beapplied to compensate for a non-uniform thickness of the liquid mediumin which the sample is immersed with respect to the path of the lightbeam as the beam rotates through 360° around the microscope objectiveoptical axis A. This non-uniform thickness may be due to anon-horizontal mounting of the sample container, or due to the presenceof meniscus of the liquid due to surface tension.

A closed loop regulation system for automatically controlling the tiltangle of the mirrors 26 a, 26 b based on feedback signals from the lightsensing system 8 may be incorporated in an electronic control circuit ofthe control system 15 of the microscope. The electronic control circuitmay also control the rotating beam mechanism 36 and the light source 4.

In the embodiment illustrated in FIG. 1b , the rotating support allowsto rotate the sample illumination beam during an image capture 360°while maintaining a fixed illumination angle α. During the rotation ofthe mirror system 34, a chosen number of image frames may be captured bythe camera, for instance in a range between 20 and 200 image frames, forinstance around 100 image frames, each representing a different viewangle of the microscopic object being observed. Each frame represents atwo-dimensional image section representing the refractive indexdistribution in a planar section of the sample being observed. The twodimensional sections may be compiled to reconstitute a three-dimensionalrefractive index based image of the observed microscopic object. Therotation speed of the beam, which depends on the rotational speed of therotating support 40 as concerns the embodiment illustrated in FIG. 1b ,may be adjusted as a function of the frame capture frequency of thelight sensing system 8.

The rotating sample illumination beam with a pre-determined illuminationangle α according to the invention is particularly advantageous overconventional solutions based on lenses, in that it allows to illuminatethe microscopic object at a large illumination angle α, as large as thenumerical aperture (NA) of the microscope objective 37 allows. Thisprovides much greater flexibility in the size of the samples that may bepositioned on the sample holder and viewed by the microscope whilesignificantly reducing the manufacturing costs of the microscope andsensitivity to the quality of the optical elements along the opticalpath. Moreover, the rotating mirror system does not change the beamshape as opposed to lenses or refractive or diffractive elements whichthus need to be manufactured with extremely high quality in order toreduce beam shaping. The latter significantly increases production costsas opposed to the solution provided by the present invention. In thepresent invention the illumination angle α may be changed, either byreplacing the rotating support 40 if the mirrors are in a fixedrelationship therein, or by having rotatable first 54 and/or second 56mirrors in the rotating support whereby the illumination angle limit isdetermined by the numerical aperture of the microscope lens. Forexample, for a numerical aperture of 0.8, the illumination angle may be55°, and with a numerical aperture 1.3 the illumination angle may be upto 64°. The higher the angle, the greater the sensitivity and thus theresolution of the two-dimensional phase image and thus thethree-dimensional phase image reconstituted therefrom.

The tilt adjustable mirrors in the optical path of the sampleillumination beam are particularly advantageous in that they allow toadjust for misalignments, manufacturing tolerances, wear during life ofthe device, and also for correction, either static or dynamic, for thevariations such as the height of the liquid in which the matter to beobserved is immersed.

Another particularly advantageous feature of the invention is theoptical path difference (OPD) adjustment device 32 having tiltadjustable transmissive or reflective light deviating elements 42, 44,30 a, 30 b, 30 c that allow for automatic and continuous calibration oradjustment of the optical path difference and also to correct for OPDwalk-off without having to change the elements. In conventional devices,a series of transparent discs of different thicknesses are positioned inthe reference beam path as a function of the adjustment of the opticalpath, however this does not allow a continuous and fine adjustment, nordoes this allow to correct for OPD walk-off.

Referring to FIGS. 7a to 7c , starting first with FIG. 7a , in a firstvariant the rotating beam mechanism 36 comprises the mirror system 34with first and second mirrors 54, 56, is mounted in a support 40 that isrotatably mounted on the housing support 12 and that is rotated through360° around the microscope objective optical axis A to perform the imagecapture of the sample 1. The second direction change mirror 26 b may beadjustable, according to a variant, or fixed, whereby in the adjustablevariant the angle of illumination a of the beam may be varied either tochange the illumination angle, or to maintain a fixed illumination angleand correct for any misalignment or misadjustment. In the second variantof FIG. 7b , the mirror system 34 of the sample illumination device isfixed, the first mirror 54 being essentially in the form of a centralcone that redirects the beam 7 b to the second mirror 56 also formingessentially an outer conical ring. The beam rotation is performed by thesecond direction change mirror 26 b of the sample beam optical pathsystem 20, the mirror performing a continuously tilting circularmovement in order to rotate the illumination beam around the cone formedby the first mirror 54 such that the illumination beam rotates 360°around the microscope objective optical axis A. The advantage of thissolution is the limited mechanical parts, however with a greatersensitivity to the accuracy of curvature of the conical mirror surfacesas compared to the mechanical solution illustrated in FIG. 7 a.

FIG. 7c also illustrates a system with a fixed support 40 whereby therotating illumination beam is actuated by a central tiltable mirror 54′that performs a circular tilting movement through 360° and that reflectsthe beam towards an upper first mirror 56 a that reflects to a secondlateral mirror 56 b.

FIG. 7d illustrates an embodiment in which the sample holder 18comprises a base 18 b and a rotating sample support surface 18 a onwhich the sample is placed. The sample illumination beam 7 b in thisembodiment is thus rotated relative to the sample by rotation of thesample holder about the optical axis A of the microscope objective 37while the beam remains in a fixed angular position. Either one or bothof the direction change mirrors 26 may be a pivotally actionable mirrorTM1, for instance in the form of a MEMS component.

Sample Illumination Procedure P1—Example of an Autocalibration Routine:

According to an embodiment of the present invention, the microscope mayadvantageously comprise an electronic control system and softwareconfigured to implement an autocalibration routine with the aim ofcontrolling the detection angle of a tiltable (e.g. MEMS) mirror 26, TM1with the aim of correcting for geometric misalignment so that the sampleillumination beam 7 b impinges upon a focal plane of the microscopeobjective 37 in the center of its field of view (FOV). This isequivalent to the incident light spot being centered on a sensor surfaceof the camera 8, respectively being centered on the captured digitalimage. The corrections are dynamic in the sense that they may changewith the rotation angle of the sample rotating beam system 36.

As the misalignments for which one would like to correct are identicalfor each rotation of the rotating beam system 36, the signal controllingthe tiltable mirror 26 a is periodic with respect to the system'srotation. For each degree of freedom i=x, y in the tiltable mirror, thesignal can be described using a countable number of discrete Fouriersine and cosine coefficients a_(n) ^(i) and b_(n) ^(i), respectively,corresponding to the rotating beam system's rotation frequency f₀ andinteger multiples nf₀ thereof. Due to the rotational character of theoptical configuration, the dominant Fourier coefficients are the DCterms b₀ ^(i), and those related to the fundamental frequency, a₁ ^(i)and b₁ ^(i), corresponding to an angular offset and a circular angularmotion of the mirror with the same frequency as the rotating beammechanism, respectively. While higher order terms may be necessary tofully describe the required correction signal, in most cases only ahandful (approx. 10) of terms is required and thus presents amemory-efficient means for storing the correction information—one which,for example, can easily be stored in the limited memory of amicrocontroller performing the real-time correction during operation.

If the incident light spot is in the objective's field of view (FOV),the spot's position can be determined quantitatively by summing thepixel values in the camera image's four quadrants. Taking the differencebetween the sums of two diagonally opposite quadrants then yields twovalues u, v that describe the spot's position with respect to the imagecenter. A spot in the center of the FOV will yield u=v=0.

The values u, v can be used as a feedback value, e.g. for a PID control.The correction signal trajectory can then be obtained by rotating thesample illumination beam relative to the sample and using the PID tokeep the spot in the center of camera's digital image. However, thisprocess may lead to the following difficulties: (i) in general, the spotis initially not in the camera's field of view and so the feedback loopis broken; and (ii) as the PID control's speed is limited by thecamera's frame rate, it may require very large frame rates to be able tofollow the spot during rotation. In other words, during rotation, it canhappen that the spot completely leaves the camera's field of viewbetween two camera frames, preventing the PID from being able to react.

To circumvent these difficulties, in an embodiment illustrated in FIG.8, a two-step process as described hereafter may be implemented. Underthe assumption that the dominant Fourier coefficients are the DC andfundamental frequencies, a first step acquires approximations of thesecoefficients so that the feedback loop must only be able to react to thehigher harmonic terms which exhibit much lower amplitudes. First,positions of the tiltable mirror centering the spot in the camera's 8field of view are found for two orthogonal static positions θ1,θ2 of therotating beam mechanism (“angles 1 and 2” in the flowchart of FIG. 9, θ1and θ2 below)—see steps P1-1 to P1-5 and P1-6 to P1-10. This is done bymoving the tiltable mirror (e.g. MEMS mirror) in a search pattern untilthe spot enters the camera's field of view (steps P1-2, P1-3 and P1-8,P1-9). A PID control is then used to center the spot in the image (stepP1-4, P1-9). The thus obtained mirror angular positions may be saved(step P1-5, P1-9) and allow the calculation (step P1-11) of the DC andfundamental Fourier coefficients corresponding to the angular offset andcircle radius and phase. This can be done by solving the 6 equations:b ₀ ^(x) a ₁ ^(x) sin θ₁ +b ₁ ^(x) cos θ₁ =x _(m1)b ₀ ^(y) a ₁ ^(y) sin θ₁ +b ₁ ^(y) cos θ₁ =y _(m1)b ₀ ^(x) a ₁ ^(x) sin θ₂ +b ₁ ^(x) cos θ₂ =x _(m2)b ₀ ^(y) a ₁ ^(y) sin θ₂ +b ₁ ^(y) cos θ₂ =y _(m2)a ₁ ^(x) =Fb ₁ ^(y)b ₁ ^(x) =−Fa ₁ ^(y)

for the 6 Fourier coefficients a and b. Here, the subscript m indicatesthe x and y values obtained from measurements for angles 1 and 2, and Fis the sensitivity ratio between the x and y directions of the tiltablemirror.

Next, the rotating beam system (for instance referring to the arm of themechanically rotated system illustrated in the embodiment of FIGS. 1a,1b ) is rotated continuously (step P1-12) and the previously determinedoffset and circular motion are dynamically applied to the tiltablemirror TM1 (step P1-13). The dominant portion of the incident lightspot's motion is thus subtracted and the residual motion is slow enoughto be followed by the feedback loop, i.e. PID control (step P1-14). Theposition may be saved (P1-15) and after a complete revolution theFourier transform (P1-17) of the feedback loop's output signal can thenbe added (P1-18) to the previously obtained Fourier coefficients (i.e.DC and fundamental frequency values) resulting in the completecorrection spectrum.

If necessary (P1-16), a second iteration can be performed, now applyingall the obtained Fourier coefficients to the mirror TM1 (e.g. a MEMSmirror) during rotation and using the feedback loop of the electroniccontrol system to again minimize the residual spot motion. This can beuseful when changing rotation frequencies as the MEMS mirror's responseis, in general, frequency-dependent and thus the coefficients may changewith the rotating beam's angular speed.

Sample Phase Correction Procedure P2—Example of Feedback Loop Control ofTiltable Mirror for Sample Based & Automated Phase Correction

This routine has the goal of finding a parameter for accessing a rangeof the lookup table (LUT) used to numerically correct a systematic errorin the phase flatness of an acquired image. The laser sampleillumination beam impinging on the sample may be slightly uncollimated.This may be due to a misalignment of the collimation optics but also dueto the diffraction-limited divergence of a Gaussian beam. When changingthe optical thickness of the sample through which this beam must travel,the curvature of the phase front in the microscope objective's field ofview (FOV) is modified in the form of a defocus (Zernicke Z₂ ⁰)aberration. It has been observed that this defocus aberration depends onthe azimuthal angle θ of the rotating illumination arm and the opticalthickness nh of the sample, where n is the sample medium's refractiveindex. As the aberration does not change from sample to sample, apre-determined LUT can be used to numerically correct the defocuseffect, given θ and nh.

The optical thickness nh of the sample can be deduced from the resultsof calibration procedure P1. It may be noted that procedure P1 may beperformed to compensate for a sample-thickness h1, h2 (see FIG. 16d )dependent walk-off of the sample illumination beam from the microscopeobjective's field of view (FOV).

As the tilting mirror TM1 positioned upstream of the rotating beamsystem is mounted statically, the rotation of the rotating beam system38 causes the correction by the tiltable mirror TM1 to be dynamic.However, the dominant component is a circular correction, correspondingto a constant angular deflection in cylindrical coordinates (α′ above).In procedure P1, the circular motion is described by the first orderFourier coefficients a₁ ^(i) and b₁ ^(i), the motion's radius given byR=√{square root over ((a ₁ ^(x))²+(b ₁ ^(x))²)}=F√{square root over ((a₁ ^(y))²+(b ₁ ^(y))²)}.

The radius R is directly proportional to the shift in the angle α′. Dueto Snell's law, there is no linear relation between nh and α′, thusbetween nh and R. However, if a look up table (LUT) is used tocharacterize the nh-dependence of correction parameter A₂ ⁰, anynonlinearity can be compensated by the LUT itself, as long as thenonlinearity was taken into account during creation of the LUT. In thiscase, the LUT parameter can be directly defined by the radius R.

The procedure steps can be listed as follow:

-   -   P2-1 After having performed calibration procedure P1, compute R        from the first order Fourier coefficients a₁ ^(i) and b₁ ^(i) as        described above.    -   P2-2 Update the corresponding LUT index from which to extract        the correction coefficients A₂ ⁰(θ).

As the defocus correction A₂ ⁰ is applied for each acquired hologramseparately, the corresponding LUT parameter θ is determined at runtime,e.g. exploiting an angular position sensor or using the hologram'scarrier frequency.

In a variant, the above procedure is performed using lower harmonics(i.e. the DC Fourier components b₀ ^(i)) and higher harmonics for thewalk-off compensation, allowing detection of a nonuniform samplethickness (e.g. a tilted dish containing liquid or a warped surface). Inthis case, the rotation symmetry above is broken and the LUT parameterR, itself, depends on the rotation angle: A₂ ⁰=A₂ ⁰(R(θ), θ).

In a variant, the optical thickness nh is determined from the opticalpath length compensation performed in P3/P4. There, the optical pathdifference induced by the sample thickness nh is compensated by a movingelement. The position of the moving element can conversely be used todeduce nh. Similarly to above, nh is nonlinearly dependent on the movingelement position, but this nonlinearity can be compensated by anappropriately calibrated LUT. Vice versa, the estimation of nh deducedby procedure P2 may be used to position the moving element in P3/P4.

Reference Beam Correction Procedure P5—Example

This routine has the goal of finding an optimal setting for tiltingmirror TM2 so that the observed holographic fringes have a frequencywhich optimizes the demodulated phase image's signal-to-noise ratio(SNR). The holographic fringes are created in the microscope bysuperimposing two light beams on the light sensing system 8 (e.gcamera), namely the object beam and the reference beam. Assuming bothbeams are plane waves with wave vectors {right arrow over (k_(r))} and{right arrow over (k_(s))}, the intensity on the camera can be writtenas:

$\begin{matrix}{{I\left( {x,y} \right)} = {2 + e^{{i{({\overset{\rightarrow}{k_{r}} - \overset{\rightarrow}{k_{s}}})}} \cdot \overset{\rightarrow}{r}} + e^{{i{({\overset{\rightarrow}{k_{s}} - \overset{\rightarrow}{k_{r}}})}} \cdot \overset{\rightarrow}{r}}}} & (1)\end{matrix}$

where {right arrow over (r)}=(x,y,z_(c)) and z_(c) is the z-position ofthe camera. While both beams exhibit a spherical curvature, the radiiare the same in both cases and can be subtracted during superposition.The reference beam's direction can thus indeed be described by a singlewave vector {right arrow over (k_(r))}. Similarly, the object beamcomprises a main component {right arrow over (k_(s))} modulated by theobject through which it has passed. The object beam's carrier frequencyfollows the direction of the rotating incident illumination, itsin-plane component describing a circle around the origin, as describedby the dotted circle in FIG. 14a . The arrow denotes the in-plane wavevector of the reference beam. The Fourier transform of the intensitypattern described in Eq. (1) is shown by the −1 order and +1 ordercircles: the constant term is displayed by the black dot near the originand the second and third summands are the circles, named “+1 Order” and“−1 Order”, respectively. In order to accurately process the holograms,the spectrum of the superimposed waves' intensity pattern (i.e. −1 orderand +1 order circles above, called the carrier frequency {right arrowover (k_(c))}) must be spatially separated from the constant termdescribed by the black centre dot. In addition, the entire circle shouldbe in a single quadrant of the spectrum, speeding up processing as onlya single quadrant must be regarded.

Introducing a tilt in the reference wave corresponds to multiplying itsexpression with a linear phase function, which direction is defined bythe vector kr. The tilted reference wave hence becomes rt=re−i(kr·x),where x=(ex,ey,ez) is the unitary vector of the Cartesian coordinatesystem:I=|o| ² +|r| ² +o*re ^(−i(k) ^(r) ^(·x)) +or*e ^(i(k) ^(r) ^(·x)),

One can identify that the interference terms are multiplied with variousphase factors, which correspond in the SFD to different modulations. Ifone considers the modulation frequency along an axis ωx parallel to themodulation direction, the inclination angle θ will induce a modulationfrequency corresponding to

${\omega_{0}} = {\frac{\sin\;\gamma}{\lambda} = \frac{\sin\; 2\alpha_{xyz}^{(2)}}{\lambda}}$

On the other hand, the detector has a sampling capacity in the xdirection corresponding to a frequency of ω=1/Δx, where Δx is the pixelsize of the camera. By considering the Nyquist theorem, the maximumangle which can be resolved by the detector is thus

${\omega_{0,x}} = {\frac{\sin\; 2\alpha_{xyz}^{(2)}\cos\;\phi}{\lambda} \leq \frac{1}{2\Delta\; X}}$$\left. {\alpha_{xz}^{(2)} \leq {2{{\sin^{- 1}\left( \frac{\lambda}{\Delta\; X\sqrt{2}} \right)}/4}}}\Rightarrow{{\sin\; 2\alpha_{xyz}^{(2)}} \leq \frac{\lambda}{\sqrt{2}\Delta\; X}} \right.$

According to this equation, for the main laser line of λ=520 nm and astandard pixel size of a CCD camera (Δx=5 μm), the maximal usable angleis Y≤4o and α(2)≤2o. In order to enable measurement of diffracted wavevectors, one can consider in first approximation limiting the angle toone half of this value, putting the carrier frequency at the center ofthe quadrant, thus yielding α(2)≤1°. Assuming a maximal band pass shiftof band pass/8, hence 512px/8=64px, the maximum shift results inΔα(2)≤0.25°.

Given a maximum distance of 300 mm from the tiltable mirror TM2 to thelight detector 8, a maximum shift results in Δα(2)≤0.25° results in alateral real space displacement of tan(0.25°)*300 mm=1.3 mm. For atypical sensor size 1024*0.005 mm=5.12 mm, a beam size of 6.4 mm shouldbe hence sufficient to adapt the carrier frequency only α(2) withoutlosing intensity in real space diameter_ref=FOV+maximum displacement.

The position of the intensity pattern's spectrum is dependent on boththe object 7 b and the reference 7 a beam's direction. As the geometryof the microscope objective 37 in relation to the camera 8 is fixed, theobject beam's direction can not easily be adjusted. The reference beam'sdirection, however, can be adjusted using a tiltable mirror, e.g. TM 2.A procedure for finding an optimal position for tiltable mirror TM2 maybe as follows:

P5-1 Calibration procedures P1 and P2 are first run through to ensurethat there are fringes observable on the light detector 8.

P5-2 Rotate the object beam 7 b by means of the rotating beam system 36(for instance rotating the support arm 40 in the embodiment illustratedin FIG. 1b ) to maximize the k_(x)-component of the object beam's wavevector.

P5-3 Compute the 2D FFT of the intensity pattern on the light detector(camera) 8 and calculate the carrier frequency {right arrow over(k_(c))} from the +1 Order.

P5-4 Compare k_(c,x) with a pre-defined minimum value k_(min). Ifk_(c,x)>k_(min), proceed to step P5-6.

P5-5 Increment the tilting mirror TM2 so that the reference beam 7 afalls more steeply on the x-axis of the camera 8. Return to step P5-3.

P5-6 Rotate the object beam 7 b by means of the rotating beam system 36(for instance rotating the support arm 40 in the embodiment illustratedin FIG. 1b ) to maximize the k_(y) component of the object beam's wavevector. P5-7 Compute the 2D FFT of the intensity pattern on the cameraand calculate the carrier frequency {right arrow over (k_(c))} from the+1 Order.

P5-8 Compare k_(c,y) with a pre-defined minimum value k_(min). Ifk_(c,y)>k_(min), proceed to step P5-10.

P5-9 Increment the tilting mirror TM2 so that the reference beam fallsmore steeply on the y-axis of the camera. Return to step P5-7

P5-10 Blank the object beam 7 b by tilting the mirror TM1 in the objectbeam path (positioned upstream of the sample 1), completely to one sideso that only the reference beam 7 a falls on the light detector 8.

P5-11 Compute the filling factor f f of the reference beam bycalculating the portion of pixels in the camera region of interest (ROI)that are illuminated.

P5-12 Compare f f with a predefined value f f_(min). If f f≥f f_(min),proceed to step P5-14

P5-13 Move the ROI away from the mean position of the non-illuminatedpixels. Return to step P5-11.

P5-14 Return the object beam tiltable mirror TM1 to its default positionto see the object beam on the light detector.

P5-15 Continuously rotate the object beam (e.g. rotating the support arm40).

P5-16 Acquire multiple holograms and compute the centroid of theircarrier frequencies as the center of the bandpass filter used fordemodulation.

P5-17 End of procedure.

Quality Assessment Procedure P6—Example Referring to FIG. 12

This routine has the goal of assessing the quality of a measurement byextracting some features of the optical beams, when no sample is presentin the field of view: intensity mean value waviness and roughness. Thegoal of this procedure is to furnish i) information on beam intensitydistributions (that can be used to diagnose a problem) and ii) a LUT forthe A_n{circumflex over ( )}m, which remain to be determined afterprocedure P2, where n≥2 and m≥1. If a sample 1 is inserted into theobject beam 7 b path, a lightened procedure must be applied to make surethat the sample is basically in the ad-hoc shape to allow proper 3Dmeasurements.

Depending on the type of observed samples, the nature of the mountingmedium and the state of cleanness of the optical surfaces, the intensitydistribution can be more or less altered. Moreover, even when beamuniformity fulfills some quality requirements, the phase of eachhologram may not be perfectly flat due to remaining slight misalignmentsin the optical paths. To allow best quality tomographic measurement, andonce the first orders aberration have been compensated, namely piston,tilt and defocus, the higher order aberrations may be quantified andnumerically compensated, as described in procedures P3 or P4.

At this stage of the calibration of the device, the procedures P1, P2,P3/4 and P5 have been conducted. We may now ensure that the full fieldof view is empty of defects which may come out as non-uniformity in theintensity distribution of the object and/or reference beam.Alternatively, as described in FIG. 18, P6 may be conducted afterapplying P1, P3/4, and P5, on a empty FOV hence yielding for a LUT ofparameters A_n{circumflex over ( )}m which are solely due to themicroscope's optical properties. Following procedure P2 serves to updateA_2{circumflex over ( )}0 which is due to the sample induced opticalproperties.

An approach is thus here to first use the tilting mirror TM2 in thereference beam 7 a optical path to mask the reference beam with respectto the light detector 8. The object beam 7 b can then be characterized.The same procedure may be carried out for the reference beam whenmasking the object beam by tilting mirror TM1. If an error is detectedat this point, the procedure P7 may be launched, otherwise, atomographic acquisition may be carried out, meaning a sequence ofholograms is acquired and analyzed.

Once the beam uniformity fulfills some predefined quality criteria, asequence of holograms, is recorded and analyzed, as previouslydescribed. From each hologram retrieved during a revolution of therotating object beam relative to the sample 1, the phase φ[θ] of thebeam signal retrieved by the light detector is calculated andcharacterized in terms again of flatness and roughness. For eachrotating beam system angle, a least-squares fit to the Zernickepolynomials R_n{circumflex over ( )}m may be calculated to produce alookup table (LUT) of the values of A_n{circumflex over ( )}mparameters. From this LUT, a flat phase can be retrieved for everyorientation of the rotating beam system and a tomographic reconstructioncan be achieved (still of an empty field of view). In addition, thismethod is prone to statistical characterization from this 3Dmeasurement, which can be compared to quality criteria in terms again offlatness and roughness.

Error Analysis Procedure P7—Example Referring to FIG. 13

When an error occurs, this procedure is launched to solve the problem ordiagnose it as much as possible to send useful information to a computernetwork (herein named the Cloud) in the form of a log file. If afterseveral testing routines, the device still does not work properly,maintenance might be required.

We only consider here failures in the above described routines from P1to P6. This error routine P7 describes how the system is able toself-diagnose a problem and solve it or gives basic and preciseinstructions to the user to do so. Two types of problems are considered:a problem linked to i) the sample, either it be the nature of the sample(absorption, thickness), or the nature of the medium in which the sampleis immersed (diffusing or exhibiting floating debris) or to ii) theoptics, for instance due to dust or a misalignment.

The approach is the following when an error is detected which preventsfrom carrying out proper tomographic measurement, or simply fromcompleting one of the procedures P1 to P6. In a first step, the deviceconnects to the cloud in order to upload a log files containingmeta-data. A second step is to carry out a blind global search in 2D ofthe object beam 7 b by pivoting of the tiltable mirror TM1. This searchis said to be blind as no specific constraints are given to restrict thesearch pattern within the range of possible tilt angle of TM1. Thisprocedure occurs either during the microscope start-up, for instancewithout sample 1, or after a sample has been inserted in the sampleobservation zone 17.

Later on, a constrained global search may be made on the reference beamtiltable mirrors TM2 to TM4, whereas constraint is given by parallelorientation of TM3 and TM4 as function of movable element or angleranges of TM2 This step aims at identifying if the problem, not solvedby the previous steps, could actually come from the reference opticalpath. The search is constrained here as the motions of the mirrors are,by design, connected by linear laws. It also allows reducing thecomplexity of the search and thus speeding it up.

After having performed those two global searches, the procedure P6 canbe done to assess the quality of the measurement. From there, theproblem is either solved or further analysis is required, which is thenperformed for different orientation of the rotating light guidingsystem. An individual and computation-intensive analysis may be appliedto the sample illumination dependent series of holograms. It aims atdefining whether the problem is orientation dependent or not.

It can then be deduced if the problem comes from the optics or thesample, and oriented instructions could be given to the user. If thequality is not satisfying yet, then the device can be remotely accessedfor more advanced analysis. If the problem remains after this procedure,the device requires special care.

FIG. 18 illustrates an example of the overall implementation of thevarious procedures P1 to P7 in the operation of a microscope accordingto an embodiment of the invention.

An amplitude and phase analysis of individual holograms captured by thelight sensing system in each of the procedures P1 to P7 described aboveis explained hereafter. The different levels of feedback can be viewedin an onion peel model, the outer surface of which describes the mostaccessible level while the innermost core is the least accessible.Access to each level first requires a successful alignment of all outerfeedback levels. In particular, the different levels of feedback controlcan be described using an onion peel model with the subsequent levelsintensity, coherence, fringe frequency and phase as shown in FIG. 14b .Each feedback level can be accessed in one or more of the presentedcalibration procedures and represents not only a monitor for thecalibration quality but also a parameter which must be optimized beforeproceeding inwards: all outer layers must be successfully calibratedbefore the next layer can be assessed. The innermost layer, the imagedphase, is finally the value used for imaging biological samples in themicroscope. The first layer, the intensity, is the least complicatedparameter to adjust as it requires no alignment of the other parameters.The intensity of the object and reference beams are optimized if thebeams pass neatly through the clear apertures of all optical elements inthe beam path. Initially, a proper alignment must be found only once,but the intensity must be adjusted each time the configuration ismodified during the alignment of the other feedback parameters. Onespecial case of intensity alignment is the calibration procedure P1, asit entails the determination of a dynamic correction that ensures thatthe intensity maximum of the object beam remains centered on themicroscope objective's FOV during rotation of the sample illuminationbeam. The intensity layer can also provide information on the sample andthe configuration of the system. For example, the coefficients obtainedin procedure P1 can give an estimate of the sample's optical thickness,as is used in P2. Also, in an embodiment where mirror TM4 is bothtiltable and translatable, the angle of the tiltable mirror TM3 in thereference beam path at which light passes through the clear aperture ofthe tiltable mirror TM4 can give an approximation of the position oftranslatable mirror TM4.

The second layer, the coherence, is optimized if the optical pathlengths of object and reference beams are shorter than the lightsource's coherence length. This is equivalent to a fixed phase relationbetween the object and reference beams and is required for interferencefringes to be observed. It is obvious that in order to obtain fringes,the intensity of both object and reference beams must be properlyoptimized, i.e. the outer layer must be successfully aligned.Conversely, after optimizing the optical path lengths, the intensityalignment should be verified. Besides providing a prerequisite forinterference, the configuration for optimal coherence is also a measurefor the optical thickness of a sample in the object beam path.

The third layer, the fringe frequency, is defined by the relative anglesof the reference and object beams incident on the light sensing system.In contrast to the intensity, this layer is influenced by the angle atwhich the optical elements in the beam path are passed through. Thislayer is optimized if the fringe frequency is large enough so that themodulated image information in the pass band around the observedhologram order does not overlap the 0 order region. The fringe frequencyis further optimized if the fringe direction is such that the carrierfrequency peak in the corresponding spectrum is in the same quadrant forall rotational positions of the rotating sample illumination beamrelative to the sample, thus speeding up processing. The properalignment of intensity and possibly also coherence must be verifiedafter modifying this layer.

The fringe frequency, i.e. the position of the carrier peak in thehologram's Fourier transform, can be used to extract useful informationat run-time, namely during acquisition. For example, the peak's maximumdefines the frequency that must be used for demodulation. The peak'sposition relative to the centroid of its rotation upon turning thesample illumination beam relative to the sample can be used to calculatethe angular position of the rotating beam system relative to the sample,to be used for look up table (LUT) based corrections of the phase.

The innermost layer, the phase of the demodulated image, may be used toreconstruct an image of the sample 1. This value is obtained byacquiring a hologram, applying a filter to the previously determinedpass band in its spectrum, and successfully demodulating the carrierfrequency. It can be seen that all outer layers must be optimized beforeassessing the phase. Given an empty sample containing the medium but notthe biological object, this parameter is optimized if the phase of theresulting image is optimally flat. This phase flatness can be degradedby a suboptimally corrected microscope objective, but also byincorrectly placed optical elements in the microscope, such as:

-   -   I. Passing a non-collimated beam through a flat window.    -   II. Passing a beam through a lens obliquely, i.e. not parallel        to the lens's optical axis.

The above aberrations, particularly no. II., can be minimized usingoptimization routines to control the beam position and incident anglesonto the optical elements. Residual phase errors can be correctednumerically if a sample-free reference is provided.

Referring to FIG. 15, phase correction for the auto calibration steps isexplained hereafter. An object scanning feedback for optical thicknessestimation is performed through the control of tiltable mirrors. Itallows sample based and automated phase correction, meaning userindependent correction without background identification, and which iscapable of dealing with high cell confluency.

The optical aberrations introduced during the imaging process can bedescribed regarding the phase transfer from object to image. It isassumed that the phase of the object φ_(o)({right arrow over (r)}) ismodified by a phase offset φ_(s)({right arrow over (r)}) of the opticalsystem. The light fields can thus be written asE _(i)({right arrow over (r)})e ^(iφ) ^(i) ^(({right arrow over (r)}))=E _(o)({right arrow over (r)})e ^(iφ) ^(o) ^(({right arrow over (r)}))=E _(o)({right arrow over (r)})e ^(i(φ) ^(o)^(({right arrow over (r)})+φ) ^(s) ^(({right arrow over (r)})))

Here, the index i denotes the image, o denotes the object and s denotesthe optical system. If the aberrations in the form of φ_(s)({right arrowover (r)}) are known, a correction phase factor φ_(c)({right arrow over(r)})=−φ_(s)({right arrow over (r)}) can be derived with which theundistorted object field can be reconstructed:E _(o)({right arrow over (r)})e ^(iφ) ^(o) ^(({right arrow over (r)}))=E _(i)({right arrow over (r)})e ^(iφ) ^(i) ^(({right arrow over (r)}))e ^(iφ) ^(c) ^(({right arrow over (r)}))

The phase offset φ_(s)({right arrow over (r)}) can be expanded in aseries of polynomials called Zernike polynomials Z_(n) ^(m), m=−n . . .n. These polynomials are depicted in FIG. 15a . Accordingly, thecorrection factor can be reduced to a set of parameters A_(n) ^(m) withφ_(c)({right arrow over (r)})=−Σ_(n) ^(m)A_(n) ^(m)Z_(n) ^(m)({rightarrow over (r)}). To numerically correct for aberrations after ameasurement, the parameters A_(n) ^(m) must be determined.

For an empty sample, namely a medium (e.g. a liquid) without biologicalsample immersed therein, φ_(i)({right arrow over (r)}) is constant andso the phase measured e.g. by a holographic microscope describes thepure aberrations. An analysis of the present aberrations indicates theirrespective origins. Z₁ aberrations, i.e. tilt, depend only on theorientations θ and α of the illumination and their magnitude candirectly be determined from the carrier frequency in each hologram. Mosthigher order aberrations (m≥2) are characteristic to the imaging systemand independent of the sample, hence these aberrations can becharacterized without sample, their corresponding correction parametersA_(n) ^(m) stored in a reference table. A special case is the defocusaberration, Z₂ ⁰, which is composed of a system-defined component and acomponent depending on the sample thickness nh and illuminationdirection. The corresponding correction parameters A₂ ⁰ must thus bestored in a reference table with additional parameters for nh and θ.

Tilt (Z1 aberrations, A_1{circumflex over ( )}(±1) factors) for eachacquired hologram can be determined directly from the hologram's Fouriertransform (special frequency domain, SFD) or through a least-squares fitin real space to the phase of a hologram frequency filtered to containonly one non-zero order (or by a combination of the two approaches). Thecorrection factors are directly proportional to the position of thehologram's carrier peak, respectively the slope of the plane fit byleast-squares.

The carrier wave frequency (respectively peak position in the SFD) canbe used to determine the angular position θ of the rotating beammechanism relative to the sample. Here, the position of the carrier peakmust be determined with respect to the centroid of the circle describedby the carrier peaks during a complete rotation of the rotating beammechanism. This center can easily be found given three carrier peaks:the centroid is the intersection of the perpendicular bisectors of theline segments connecting the three peak positions. Given the rotatingbeam mechanism position θ, the remaining correction factorsA_n{circumflex over ( )}m can be obtained by inserting θ (and the samplethickness) into a lookup table. This lookup table is previouslypopulated from calibration measurements on transparent, homogeneoussamples such as in procedure P6. For various sample medium thicknessesnh and rotating beam mechanism angles θ, the uncorrected phase ismeasured and a least-squares fit to the Zernicke polynomialsR_n{circumflex over ( )}m yields the lookup table values.

Referring to FIGS. 15b and 15c , typical measurements, represented byhigher order development as function of time (during rotation), ofastigmatisms and defocus in different sample conditions is illustrated.In FIG. 15b , the sample is provided by a coverslip for which defocus isonly a function of rotation, the magnitude of the defocus' offset isdetermined by the optical thickness, and astigmatism is constant in timeand dependent on rotation. In FIG. 15c , the sample is in a Petri dishwhereby defocus may be a function of rotation and evaporation (changingin time), the magnitude of the defocus' offset is determined by thesample's optical thickness and a possible meniscus, and astigmatism isconstant in time and dependent on rotation.

These measurements tell us that the mean magnitude of defocus varieswith the sample's optical thickness. Therefore, the goal of a scan byvarying the angle of a tiltable mirror in the sample illumination pathis to determine the sample's optical thickness and by these means themagnitude of defocus. In case of an aqueous solution, a meniscus mayintroduce additional defocus. For best defocus estimation, a measurementof eg theta dependence (during calibration) of defocus could serve asmeniscus estimation (amongst others).

Referring to FIGS. 16a-16c , an estimation of the pre-factor mentionedabove is explained. During a scan in which the angle of the tiltablemirror is varied, measurement of the light beam signal captured by thelight sensor (e.g. camera) of the light sensing system provides thefollowing information: the intensity in the center of the field of view,i.e. at (x=0), will show a maximum if the incident light's angle α issuch that the light beam falls directly into the field of view. This isthe case displayed in FIG. 16a for the solid line. For a varying opticalthickness thickness nh of the sample, this maximum will shift to adifferent α, as displayed in FIG. 16b . The relation between the angle αand the optical thickness can be given using Fermat's principle andinvolves minimizing the optical path ψ for a given geometry:ψ=n ₂√{square root over (h ² +x ₁ ²)}+n ₁√{square root over ((y ₀−h)²+(x ₀ −x ₁)²)}.

Here, (x₀, y₀) is the position of the beam reflected off of the tiltablemirror and (x₁, y₁) is the position of the beam entering the samplemedium. A nonlinear relation between α and the optical thickness nh canthus be derived, but for optical thicknesses small compared to thedistance to the mirror, the response is nearly linear, as shown in FIG.16c for typical values in the microscope.

$\psi = {\sqrt{n\; 2^{2}\left( {h^{2} + {x\; 1^{2}}} \right)} + \sqrt{n\; 1^{2}\left( {\left( {{x\; 0} - {x\; 1}} \right)^{2} + \left( {{- h} + {y\; 0}} \right)^{2}} \right)}}$${d\;\psi} = {{\frac{n\; 2^{2}x\; 1}{\sqrt{n\; 2^{2}\left( {h^{2} + {x\; 1^{2}}} \right)}} + \frac{n\; 1^{2}\left( {{{- x}\; 0} + {x\; 1}} \right)}{\sqrt{n\; 1^{2}\left( {\left( {{x\; 0} - {x\; 1}} \right)^{2} + \left( {h - {y\; 0}} \right)^{2}} \right)}}} = 0}$

Example of reduction of tolerances on mechanics and compensation of heatdiffusion through the use of tiltable mirrors:

Referring to FIG. 17a , an illustration of the sample illumination beamrotating head architecture is represented to study influences ofmechanical tolerances discussed below. Changing parameters include:

-   -   alpha: angle of the central mirror 54 with respect to the        optical axis A.    -   beta: angle of the rotating beam system 28 with respect to the        structure.    -   theta: angle of peripheral mirror 56 with respect to the optical        axis in neutral position.    -   d: position of central mirror 54 in its normal direction.    -   d′: distance between the two mirrors 54, 56.    -   d″: horizontal position of the arm compared with the hollow        shaft 58.

A chosen criterion is to be within a certain tolerance of the MEMSmirror 26 b compensation angle, which may be for instance 0.1°,corresponding to a static error of its normal direction about itsmechanical pivot axis.

Different combinations of errors have been studied and results are shownin the tables of FIGS. 17b -1, 17 b-2, and 17 b 3.

Tolerances that are aimed for may be for instance:

-   -   d/d′ and d″: 0.1 mm (not very critical)    -   alpha/beta/theta: 0.025° corresponding to 0.0055 mm for the        geometrical tolerances.

As previously described, the tiltable transmissive or reflectiveelements of the OPD system may also be controlled by means of a feedbackloop based at least partially on the light beam signal captured by thelight sensing system to adjust and correct the path of the referencebeam.

More generally, an important aspect of the invention is the use offeedback from the beam signal captured by the light sensing system toautomatically control, by means of the electronic control system, theangle of the one or more tilt adjustable mirrors by the control systemin order to correct for unwanted deviations of the sample illuminationbeam or the reference beam, or both the sample illumination andreference beams, or to create wanted deviations (e.g. to control theOPD). Deviations in the sample illumination path may be due tomanufacturing tolerances, wear of components of the microscope over itslifetime use, thermal dilatation effects, or variations in the samplemedium such as liquid height, meniscus curvature, non-horizontal liquidsurface and other factors. Deviations of the reference beam may also bedue to manufacturing tolerances, wear of components of the microscopeover its lifetime use, and thermal dilatation effects, and also desireddeviations to control the optical path difference. The control of thesedeviations in either the sample illumination beam or the reference beamby reading the beam signal captured by the light sensing systemadvantageously provides an automated manner to obtain high qualityimages in a simple, easy and cost effective manner with minimal setup.

Dynamic OPD Procedure P8—Example Referring to FIGS. 5f, 5g and 2c, 2d

Due to defects or simply imperfections in the mechanics, the length ofthe optical path (OPL) of the sample beam is likely to vary when therotating scanning arm is completing a 360° rotation during theacquisition of the holograms. If those defects result in a OPL variationconfined to a fraction of the coherence length of the laser, theprocedure described in relation to FIGS. 4h-4i may be used to haveoptimum fringe contrast for any orientation of the scanning arm (seeFIGS. 2c and 2d ).

However, this defect may result in a variation of the optical pathlength beyond the coherence length reducing thus greatly or evencancelling the fringe contrast in certain orientations of the arm,making the phase calculation meaningless in those positions.

To compensate for this and find the minimal OPL as illustrated in FIG.5g , the OPL is dynamically adjusted by the control system of themicroscope to keep the OPD between the reference and sample beams farbelow the coherence length of the laser source and ensure an optimumfringe contrast along the rotational scanning process.

The method for defining the parameters that may be applied to the OPDsystem 32 synchronously with the rotating beam system 36 comprises thefollowing steps:

-   -   a) place a sample on the sample holder,    -   b) position the rotating beam system in a first position,    -   c) position said at least one pivotally actionable direction        change mirror (TM3, TM4) configured to direct the reference beam        in a first position,    -   d) measure a position of the reference beam signal captured by        the light sensing system while the sample beam is switched off,    -   e) switch on the sample beam and measure a fringe contrast of a        signal captured by the light sensing system    -   f) change by an increment the position of said at least one        pivotally actionable direction change mirror (TM3, TM4),    -   g) repeat steps d) to f) until the sum of increments corresponds        to a pre-defined working range of the pivotally actionable        direction change mirror (TM3, TM4),    -   h) compare the fringe contrast measurements obtained for each        increment and store in look-up table (LUT) of a memory of the        control system the position of the pivotally actionable        direction change mirror (TM3, TM4) for the fringe contrast        measurement with the highest value, in conjunction with the        position of the rotating beam system;    -   j) rotate by a small increment the rotating beam system and        repeat steps c) to h) until the rotating beam system has        completed a 360° rotation.

A look-up table (LUT) may thus be created to provide the parameters thatcan be applied to dynamically control the OPD system 32 synchronouslywith the rotating beam system 36.

By way of example, this procedure is illustrated in the embodiment ofFIG. 5f , which comprises the following steps:

-   -   Once a sample is placed on the XY stage (step P8-1), the        rotating arm 40 is placed in its parking position (step P8-2)    -   The procedure P4 (illustrated in FIG. 5e ) is then launched        (step P8-3) to find the optimum OPD position for this arm        position leading to the maximum fringe contrast.    -   The OPD system parameters are saved for this arm position (step        P8-4).    -   The scanning arm is then rotated by a small increment (step        P8-5) and the procedure P4 is then launched again. The        parameters are again stored.    -   The previous steps P8-3 to P8-5 are repeated until the arm is        back to its parking position.    -   A look-up table (LUT) is thus created (step P8-6), gathering the        parameters that must be applied to the OPD system 32        synchronously with the rotating beam system 36.

List of references microscope 2 (coherent) light source 4 (laser)Reference beam 7a Sample illumination beam 7b (also named herein objectbeam) light beam guide system 6 beam splitter 14 sample beam opticalpath 20 direction change mirrors 26, 26a, 26b tiltable mirrors TM1(MEMS) sample illumination device 28 mirror system 34 first mirror 54second mirror 56 rotating beam system 36 drive 38 transmission 39 (belt)support 40 (rotating arm - embodiment FIG. 1a, 1b, 1d, 7a,/fixed-embodiments FIG. 7b, 7c) actionable mirrors hollow axis 58 bearings 59mirror support body 60 pivoting beam configured to rotate microscopeobjective 37 reference beam optical path 22 direction change mirrors 30tiltable mirrors TM2, TM3, TM4 (e.g. MEMs mirrors) mirror surface 61pivot axis 63 optical path difference (OPD) adjustment device 32 firstlight deviating element 42 second light deviating element 44 pivotsupports 46, 48 pivot axis 49, 51 teeth 50 lens 65 (also named hereinfield lens) beam reuniter 16 light sensing system 8 (detector, camera)image data processing system 10 housing/support structure 12 sampleobservation zone 17 sample holder 18 height adjustment mechanism samplelight control system 15 mirror (e.g MEMS mirror) tilt angle controlMirror rotation drive control Source control Camera control sample 1closed containing system 3a base 11 coverslip 9 observation plane 13seal 7 open dish 3b buffering medium 5

The invention claimed is:
 1. A microscope comprising a light sourceproducing a collimated light beam, a light beam guide system comprisinga beam splitter configured to split the light beam into a reference beamand a sample illumination beam passing through the light beam guidesystem directed by at least one direction change mirror being pivotallyactionable (TM1, TM2, TM3, TM4) to guide the reference beam and samplebeam along their respective optical paths, a sample observation zoneconfigured to receive a sample to be observed in a path of the sampleillumination beam, a sample illumination device configured to direct thesample illumination beam through the sample observation zone and into amicroscope objective, a beam reuniter configured to reunite thereference beam and sample illumination beam after passage of the sampleillumination beam through the sample observation zone, a light sensingsystem configured to retrieve at least phase and intensity values of thelight beam downstream of the beam reuniter, and a control system,wherein the sample illumination device comprises a mirror systemconfigured to direct the sample illumination beam at a non zeroillumination angle (α) with respect to an optical axis (A) of themicroscope objective, and a rotating beam mechanism configured to rotatethe angled sample illumination beam around the optical axis.
 2. Themicroscope according to claim 1, wherein the mirror system of the sampleillumination device is mounted in a rotating support and the rotatingbeam system is formed by the rotating support and a motor drive torotate the support.
 3. The microscope according to claim 2, wherein therotating support comprises a hollow axis configured to allow the sampleillumination light beam to pass therethrough, the hollow axis mountedvia bearings to a housing support structure.
 4. The microscope accordingto claim 2, wherein the rotating support comprises a mirror support bodyin which first and second mirrors are mounted.
 5. The microscopeaccording to claim 4, wherein the first mirror redirects the samplelight beam projected through the hollow axis onto the second mirror, andthe second mirror redirects the sample light beam onto the samplepositioned on the sample holder at an illumination angle (α).
 6. Themicroscope according to claim 4, wherein at least one of the first andsecond mirrors is a tilt actionable mirror controlled by the controlsystem.
 7. The microscope according to claim 5, wherein the first andsecond mirrors are arranged in a fixed relation within the mirrorsupport body.
 8. The microscope according to claim 1, wherein therotating beam system comprises rotating tilt actionable mirrors todirect the sample illumination beam on the mirror system of the sampleillumination device and wherein the mirror system is mounted on a fixedsupport.